Thermostatic control for multipass evaporators



Aug. 29, 1939. I L. SH-RODE 7 07 THERIQSTATIC CONTROL FOR MULTIPASSEYAPORATORS Filed May 14, 1936 3 Sheets-sheaf 1 COMP/M1530 J. L. SHRODETHERIOSTATIQ CONTROL FOR MULTIPASS EVAPORATORS 3 Sheets-Sheet 2 Filedlay14, 1936 Au 29, 1939. J, L, SHRODE 2,171,407

THERIOSTATIC CONTROL FOR MULTIPASS EVAPORATORS Filed May 14, 1936 3Sheets-Sheet 3 LIL. 519127415 Patented Aug. 29, 1939 UNITED STATESTHERMOSTATIG CONTROL FOR. MULTIPAss EvAPonAToRs John L. Sin-ode, St.Louis, Mo., assignor to Alco Valve Company, Incorporated, St. Louis,Mo., a corporation of Missouri Application May 14,

' 4 Claims.

This invention relates to the control of multipass evaporators and ithas for its general object-the provision of means, in Vaporizers of thedirect expansion type, for close and flexible control of the .supply ofrefrigerant to individual tubes or groups of tubes, for maintaining anoptimum value of thermal efficiency in all elements of the evaporatorthroughout the entire load range, and with regard to the variable loadincident to the location of different parts of the evaporator.

Most specifically, the invention relates to a control system formulti-pass evaporators, em-

. ploying independent expansion valves for each tube or for groups oftubes, and controlled by thermostatic means-, likewise individual to thetubes or groups of tubes and regardless of whether the tube loops 'of acontrolled unit lie in one or a plurality of planes, said thermostaticmeans being located at the suction end of the evaporator and quicklyresponding to a temperature of superheat at suction pressure justsufliciently high to prevent refrigerant in liquid form from being drawnover to the compressor.

Other objects of the invention will appear as the following descriptionof a preferred and practical embodiment thereof proceeds.

It is of course a matter of elemental knowledge that the capacity of arefrigerant to absorb heat is of consequential magnitude only while theliquld is changing state, and that after the liquid has become a gas ithas very little absorptive capacity. The saturation point of the gas is,therefore, the limit of its thermal efficiency. To obtain the maximumefficiency from a given length of evaporator coil, it is thereforevnecessary 'to admit the liquid refrigerant to the coil at such a raterelative to the refrigerating demands that it will be changing its stateclear to the posterior end of the coil, that is to say, the end remotefrom the expansion valve.

A number of practical factors generally supervene to prevent realizationof this ideal condition, for instance, the control means may be sluggishin responding to load variations. If the load increases, the saturationpoint may shift from the posterior or suction end of the coil, towardthe anterior end of the coil. The length of coil between this saturationpoint and the suction end of the evaporator may be regarded assubstantially functionless in the performance of useful refrigerationsince it is occupied by gas above the saturation temperature and withvery little heat absorbing capacity. If the load should suddenlydiminish, the saturation point would move rearward, altogether out ofthe coil and toward the compressor. Since it is very important that theliquid refrigerant be prevented from passing over to the compressor, thecontrol means is ordinarily so set as to as- 1936, Serial No. 79,765

sure that the saturation point will remain in the coil under thelightest load, and under such setting of the control mechanism, itfollows that under heavy load the saturation point advances quite amaterial distance toward the anterior portion of the coil. Therefrigerant gas to the rear of the saturation point in the coil, due toits small mass and density quickly" rises in temperature, toward thetemperature 'of the cooled medium outside the coil and this rise, overthe temperature at the point of saturation is known as superheat.

As stated, a certain amount of superheat is essential to prevent liquidbeing drawn over into the compressor, which, stated in other words,means that a certain length of the coil must be devoted to thesafeguarding of the compressor and thus withdrawn from functioning in arefrigerating capacity. A further amount of superheat must be developedto create enough pressure in the thermostat to operate the diaphragm ofthe expansion valve against the tension of its closing spring, whichmeans that a still further length of the coil shall be devoted to anonrefrigerating purpose.

Asid from these inherent negative factors, the efficiency of a singlecoil may be maintained at an optimum value by placing the thermostaticcontrol at the. posterior end of the coil where it will respond to thesuperheat temperature at suction pressure, and employing the samerefrigerant in the thermostatic capsule as is used in the refrigeratingsystem. Such expedients of control are known.

In a multi-pass evaporator however, a different situation is confronted.It is seldom that load conditions are idential for all the passes andconsequently, a single expansion valve for the entire evaporator with asingle thermostatic control is incapable of determining maximumefficiency in all of the passes. The present invention seeks to providea control in a multi-pass evaporator which will establish uniform andoptimum efficiency in all of the passes regardless of the load range andregardless of variations in load for the several passes.

Referring now to the drawings in which similar characters of referencehave been employed to designate identical parts of the several views:

Figure l is a front elevation of a multi-pass r show a section on alower plane of a modified form of the invention;

Figure 4 is a. section taken along the line 4-4 of Figure 3;

Figure 5 is afragmentary view showing an alternative means forpartitioning off a conduit, in the exercise of the invention;

Figure 6 is a rear view on a reduced scale of the evaporator shown inFigure 1, parts being shown in section;

Figure '7 is a front elevation of an evaporator in which all of the tubeloops under a single control are in one plane; and

Figure 8 is a side elevation of the evaporator shown in Figure 7.

Referring now in detail to the several figures, and first to that formof the invention shown in Figures 1, 2, 5 and 6, the numeral Irepresents in general an evaporator employed in air conditioning andconsisting of an upper header 2 and a lower header 3 and the tube loops4. The arrow shown in Figure 1 shows that the air is blowing against theevaporator perpendicularly to the planes of thepasses. This is only byway of example for the air or other medium to be cooled might flow inany relation to the evaporator coils without transcending the spirit andscope of the invention.

For convenience in illustrating the invention, the passes have beendivided into groups of four, each group being hereinafter referred to asa tube loop unit, said unit consisting of the number of tubes served bya single expansion valve including those headers or those segregateparts of a header which the opposite ends of the tubes are attached andwhich functions solely in serving that group of tubes to which they areattached. It is obvious from Figure 1 that the group a bears the bruntof the cooling since the air strikes it in an entirely unrefrigeratedcondition and that the groups b and c carry lesser loads in the ordernamed for the air has been partially cooled after passing the firstgroup and cooled still further after passing the second. It is obviousthat under such conditions a single expansion valve and a singlethermostatic control could not regulate the admission of refrigerant toall of said passes in such a manner as to assure that the saturationpoint of the refrigerant would be uniformly close to the posterior endsof each of the passes. If the thermostat and the expansion valvedetermined a correct control for the passes of the group a, this wouldnot be the correct control for the passes of the groups I) and c whichare subject to smaller loads and refrigerant in liquid form, would bedrawn from the groups b and 0 back to the compressor.

The present invention contemplates dividing the upper and lower headers2 and 3 into a plurality of segregate chambers 5, 6 and I by partitions8 and 9 and in providing independent expansion valves l0, II and I2 [oreach of the upper chambers, and independent thermostats I3, -14 and I5for each of the lower chambers. Thus the tubes of groups a will be underthe control of one expansion valve thermostat couple while groups I) and0 will respectively be under the control of other independentexpansion-valve-thermostat couples, each couple being responsiveto thesuperheat of the effluent refrigerant-in the particular chamber withwhich the thermostatic element is associated. The several expansionvalves are supplied from a conduit l6 leading from the high side of thecompressor, while the thermostatic elements are in the branches l1, I8and IQ of a common eiiiuent conduit 20 leading to the low side of thecompressor. It is, of course,

conceded that the individual tubes of each of the groups are subjectedto different load conditions so that the control of a group of tubes orpasses by a single expansion valve and thermostatic element is merely anapproximation of optimum efiiciency, and if it be desired to carryeflieiency value still higher, the groups could be still furthersub-divided even to the extent that an independent valve andthermostatic bulb would be provided for each tube. Of course, theexpense of equipment would be a prime consideration in determining howfinely the independent control of the several passes shall besub-divided.

Inasmuch as the invention contemplates the independent control of eachpass or group of passes, it is imperative that the remote thermostaticelements for the several valves be so arranged that the valve responsewill be a result of the change in condition in the section of theevaporator which the valve is controlling. The usual method of controlwith the thermostatic bulbs strapped to the outside of the suctionconnection from the various sections, introduces many variables whichprevent the accurate and positive response of the valve to thecontrolled condition. The mass effect of the refrigerant conduit, theconductivity of the metal in the refrigerating conduit, and theradiation from the outside atmosphere surrounding the bulb, all tend tointerfere with the correct transfer of heat at low temperature gradientsfrom and to the thermostatic bulb. A further objection to this mostcommon method of bulb application (strapped to the suction line) is theconductivity of the suc tion line which causes the suction gas from onesection to change the temperature gradient on the thermostatic bulbs ofone or more of the other valves, thus creating an unwarranted increaseor decrease in refrigerant flow through the sections whose valves werespuriously affected.

With the present invention, the suction connections from the variouscoil sections are connected 'to common manifold sections so constructedthat liquid refrigerant will not trap, and so that the isolatedthermostatic bulb l3 may be inserted in the separate suction conduits H!from each section. It will be understood that in the preferred for ofthe invention, the bulbs l3 are completely insulated from the walls ofthe surrounding conduit not only by their spaced relation from thelateral walls of said conduit, but by the interposition of an insulatingspace and support 2| at their base.

It is obvious that the use of the insert type of thermostatic bulbwithin the refrigerant conduit provides for a control which responds tothe temperature of the refrigerant vapor and minimizes the mass andconductivity of the suction line and the necessary fittings as adisturbing factor. It will be recognized that it is only with this typeof thermostatic bulb that a speed of response necessary to'preserveminimum superheat with freedom from hunting or cycling is -malntained,and without the dang-er of back flooding.

tubes would make it diflicult to find room for the installation of theplurality of expansion valves and thermostatic elements.

, Figures 5 and 6 suggest the use of partitions iii and 3| blocking thecontinuity of the liquid and suction conduits IB and 2D, adapting thesystem to the use of multiple compressors, or the partitions may beemployed for obtaining capacity reduction in conjunction with stopvalves 35 and 36 placed on one or both sides of the liquid conduit 6,for example, even when the two sides of the conduit are connected to thesame compressor.

In Figure 5 a simple expedient for installing the partition 3| issuggested. The partition 3| comprises adisk having a concave periphery32 arranged in the conduit l6 perpendicular to the axis thereof andhaving the conduit peened or upset inwardly forming beads which seat inthe channel in the partition [6 and maintain it permanently in position.

Referring now to Figures 7 and 8, a form of invention is shown in whichthe coils of each controlled unit lie in a single plane. As shown, theunit consists of three groups of coils 36', 3'! and 38 arranged onebeneath the other. Where coil sections are thus vertically arranged, aproblem arises regarding the uniform distribution of liquid refrigerantto each of the groups. This is accomplished in the present instance bythe provision of a common liquid manifold 39 having independent tubularbranches 40, 4| and 42 leading to the influx ends of the several coils,and a vertical suction manifold 43 to which the several effiuent ends ofthe coil sections are connected at different levels. Since themanifold39 as illustrated in Figures 7 and 8 is common to threecontrolled units, it is preferred to partition off said manifold betweensaid units as by the partitions 44 and 45. The manifold 39 is suppliedwith liquid refrigerant from a conduit 46 to which the individualexpansion valves 41, 48 and 49 are connected. Said expansion valves alsocommunicate with the individual sections of the manifold 39 definedbetween the partitions 44 and 45.

Thermostatic bulbs 50 individual to the suction manifolds 43 areimmersed in said manifolds preferably at the top, each being connectedby a tube 5|. Thus individual control of the evaporator coils which liein one plane is accomplished, the dividing of the coils into groupsbeing primarily in the interest of uniform distribution.

In Figures 3 and 4, another form of the invention is illustrated inwhich the evaporator tubes 22 emanate from a common manifold 23 which isdivided into isolated sections by an insert consisting of a rod 24having disks 25 and 28 secured in perpendicular relation thereto andhaving an end piece 21 secured in like manner. The disks 25 and 26 areproperly spaced so that when the insert is in place they isolate thesections of the manifold commanded by the thermostatic devices 28, 29and 30. This construction is particularly adapted to the conversion ofexisting evaporators into apparatus embodying the principles of thepresent invention and in adapting the several sections of the manifoldto be connected to separate compressors if desired.

While I have in the above disclosure described what I believe to bepreferred and practical embodiments of my invention, it will beunderstood to those skilled in the art that the specific embodiments asshown, and the details of construction as illustrated and described aremerely exemplary and that the inventive principle may be applied to anygrouping of tubes in an evaporator, no' matter how they may be arrangedwith respect to the liquid and suction headers.

What I claim is:

1. Refrigeration system including in combination with a multi-passevaporator having tube loop units connected in multiple between theconduits which constitute the liquid and suction limbs of said system,the suction limb conduits constituting branches of a suction manifold,means for maintaining a maximum area of wetted surface in each tube loopunitregardless of variable loads on the several tube loop units,comprising an expansion valve and a control thermostat individual toeach tube loop unit, the thermostats being located in said brancheswhereby they respond to the temperature of the eflluent gas adjacent thesuction ends of the respective tube loop units.

2. Refrigeration system including in combination with a multi-passevaporator having tube loop units connected in multiple between theconduits which constitute the liquid and suction limbs of said system,the suction limb conduits constituting branches of a suction manifold.means for maintaining a maximum area of wetted surface in the severaltube loop units of the evaporator regardless of variable loads on theseveral tube loop units, comprising expansion valves individual to theseveral tube loop units, and thermostats for controlling said expansionvalves individual to the several tube loop units, said thermostats beingimmersed in the efiiucnt gas adjacent the suction ends of the respectivetube loop units, whereby they are substantially isolated from thethermal conductivity of proximate metal masses.

3. Refrigeration system including in combination a multi-pass evaporatorcomprising a plurality of tube loop units, headers to which said tubeloop units are connected in multiple, said headers being divided intosections individual to a tube loop unit and forming parts of such unit,conduits constituting the liquid and suction limbs of said system,branch conduits individual to said sections connecting the respectiveheaders to the liquid and suction conduits, expansion valves in dividualto the header sections located in the branches to said liquid conduitand thermostats for controlling said expansion valves, individual tosaid header sections and locateddn the branches communicating with thesuction conduit of said system, said thermostats being immersed in theciiiuent gas within said branches.

4. Refrigeration system including in combination a multi-pass evaporatorcomprising a plurality of tube loop units, inlet and outlet headers towhich the opposite ends of the loops of said units are respectivelyconnected, conduits constituting the liquid and suction limbs of saidsystem, expansion valves individual to the tube loop units communicatingwith the liquid conduit and with the header on the high side ofsaidunits, branch conduits individual to said tube loop units communicatingwith said suction conduit and with the header on the suction side ofsaid evaporator and thermostatic bulb elements individual to said tubeloop units within the branches of said suction conduit and operativelyrelated to said expansion valves.

JOHN L. SHRODE.

